The metasediments of the Schist-Greywacke Complex (Durico-Beirao Supergroup) of late Neoproterozoic to lower Cambrian age are at present divided in Portugal into Beiras, Douro and Arda-Marofa Groups. In the Sierra de Lousa (western part of Central Portugal) there are outcrops of metasediments which belong to the Beiras Group, which comprises two formations known as Caneiro and Boque-Serpins. This work concentrates on the Boque-Serpins formation at Trevim, which is constituted by a dominant metapelitic succession that sporadically contains metagreywacke alternances. The great variety of sedimentary structures, most of them interpreted as wave generated helps define a set of facies associations which are arranged in several types of sequences reflecting a wide range of energy conditions that are explained as resulting from the interplay of storm and fear weather conditions. The sedimentolcgical characteristics recorded are consistent with a muddy shallow marine platform depositional environment on which longshore directed sandy bars were formed under the influence of storm events. On the other hand, the diversity of Quartz and Rock Fragments found in the greywackes has made it possible to identify several source areas (sedimentary, metamorphic, plutonic and volcanic) which, given their metaestable character and degree of preservation, would be located near the sedimentary basin